The present disclosure relates to an image forming apparatus such as a copier, a printer, and a facsimile machine, and relates particularly to an image forming apparatus capable of compatibly incorporating two kinds of developing devices employing different developing methods.
The following methods are included in conventionally known developing methods involving use of dry toner and employed in an image forming apparatus using electrophotographic process: a one-component developing method in which no carrier is used, and a two-component developing method (hereinafter also referred to as a first developing method) in which a two-component developer including a magnetic carrier (hereinafter also referred to simply as carrier) and non-magnetic toner that is electrostatically charged by the magnetic carrier is used to develop an electrostatic latent image on a photosensitive body by using a magnetic brush of the toner and the carrier formed on a magnetic roller.
Further, in cases of color printing, where color superimposition is performed, color toner having transparency needs to be used, and thus the color toner is required to be non-magnetic toner. For this reason, full-color image forming apparatuses often adopt the two-component developing method where toner is conveyed and electrically charged by using a carrier.
On the other hand, there has been proposed a developing method (hereinafter referred to also as a second developing method) where, when a developer is transferred, by using a magnetic roller (a developer carrier), onto a developing roller (a toner carrier) disposed out of contact with an image carrier (a photosensitive body), non-magnetic toner alone is transferred onto the developing roller to form a thin toner layer thereon, with a magnetic carrier left on the magnetic roller, and the toner is caused to adhere to an electrostatic latent image on the image carrier (the photosensitive body) by an alternating-current electric field.
There has been also proposed a developing device where a developer including a carrier is supplied into a developing container and surplus part of the developer is discharged, to thereby reduce degradation of the charging performance. A known example of such a developing device is one that is provided with supply means that performs additional supply of a developer via a supply port formed in a developing container, and a developer discharging mechanism that discharges the surplus part of the developer to outside the developing container.